Thermo-fluid-dynamics of the evolution of the lamella thickness of a droplet impacting onto a heated substrate

A high speed droplet impact onto a dry substrate at ambient temperature begins as a sudden impulse, and is followed by a smooth inertia-dominated phase and a viscosity-dominated phase. Similarly, droplet impact onto a heated surface begins as a sudden thermal shock at the impacting end of the droplet, followed by a more gradual heat transfer process that is coupled with the evolving flow field and changing shape of the droplet. The present experimental study investigates the impact dynamics of water droplets on heated substrates, focusing on the gradual heat transfer process within the droplet and its effect on the lamella thickness and droplet spreading, as the droplet shape changes during the impact. The Weber numbers used in the study range from 197 to 604 and the surface temperatures from 25 °C to 200 °C. The total heat transfer up to the maximum spreading point is estimated, along with the induced temperature increase in the droplet. It has been observed that the kinematic and inertia-dominated phases of droplet impact remain unaffected by heat transfer. The influence of heat transfer becomes apparent only when the droplet transitions into the viscosity-dominated phase. A Chromatic Confocal Sensor (CCS) is used to measure the transient lamella thickness near the point of impact. The measured thicknesses indicate that when the thermal boundary layer thickness surpasses the lamella thickness, vaporization becomes dominant, leading to rapid thinning of the lamella, leading to break up and dewetting, particularly at high Weber numbers and elevated substrate temperatures.